The field of art to which this invention pertains is pigment dispersions particularly dispersions of transparent pigments.
Transparent pigments have been utilized in "metallic" automotive coatings for quite a number of years. The preparation of such coatings using transparent red oxide pigments is described in U.S. Pat. Nos. 2,335,760, 2,384,579 and Re. 23,757. The so-called transparent pigments have extremely small particle sizes which exist in agglomerated form, the less the agglomeration the more the transparency. U.S. Pat. No. Re. 23,757 describes a process of "flushing" pigments from the wet pulp stage directly into lacquers without the intermediate step of drying and pulverizing the pigment. When properly conducted, the very small primary particles of the pigment do not agglomerate to any great extent and do not change markedly from their freshly precipitated condition. However, this "flushing" procedure has disadvantages. The pigments must be used in a freshly precipitated condition which requires the pigment manufacturing facilities to be in close proximity to the paint making facilities. Shipping costs are high since water must be shipped with the pigments. After the dispersion of pigment in the paint has been made, the water must be removed from the paint.
When the precipitated pigments are dried before use, they form agglomerates which are extremely difficult to break up. Simply grinding and pulverizing the pigments do not work. Some success has been obtained in dispersing the pigments in resin solutions using various dispersion means, e.g., roll mills, steel ball mills, sand grinders, high speed dispersers and the like. However, completely satisfactory pigment dispersions having long term stability and which product coatings having a bright face, clean color and excellent transparency have not been obtained by these methods.